Metallurgical processes, including steel processes, result in waste by-products of iron and steel dust. There have been many attempts to recover the Zn and Pb in this dust and to obtain a by-product which can be safely recycled or which can be safely buried with minimal metal leaching problems.
The Waltz kiln has been employed to recover Zn and Pb using a reducing roasting process. The Waltz kiln which is used is a directly heated counterflow type rotary kiln. The reducing roasting process comprises directly heating and roasting the iron and steel dust in the Waltz kiln in a reducing atmosphere under suitably selected conditions of temperature and retention time, thereby separating Zn and Pb through volatilization from the dust and enabling iron to be discharged in the form of solid reduced iron aggregates. In practice, the reducing roasting process using the Waltz kiln is problematic because it is extremely difficult to maintain the operation of the rotary kiln under the appropriate conditions for a long time and a retention time of more than 1 hour is generally required. The recovery of Pb and Zn by this process is not satisfactory. Because of direct heating by combustion of reductant and high velocity gases containing uncontrollable CO.sub.2 and O.sub.2 contents, unwanted particles are carried over into the zinc concentrate resulting in a poor quality of zinc concentrate. For satisfactory results the zinc has to be processed in two stages.
With the reducing roasting process using the Waltz kiln there is also a fluctuation in the operating conditions due to the deposition of low melting compounds on the walls of the kiln. This consequently impedes the continued operation of the rotary kiln. As a result, efficiency of zinc recovery is low. Although, it is possible to reduce the effects of this problem be adding the flux to the feed to the rotary kiln, thereby adjusting the melting point of the feed and enabling the feed to be completely melted within the rotary kiln.
U.S. Pat. No. 4,525,208 attempts to avoid the above problems with the Waltz Kiln caused by the depositing of material on the walls of the rotary kiln by running the volatilization in two stages. In the first stage the material is heated and Zn and Pb are partially evaporated at a lower temperature in a rotary kiln. In the second stage, the solid material from the rotary kiln is continuously fed into the rotary smelting furnace where fluxes are added to the material to lower the melting point facilitating the evaporation of the metals from the molten stage.
Other improvements in recovering zinc and lead are described in U.S. Pat. No. 3,850,613 and in U.S. Pat. No. 5,013,532. In U.S. Pat. No. 3,850,613 the improvement for separating and recovering the Zn and Pb includes granulation and bricketing the dust and then volatizing the Zn and Pb by heating the briquettes. In U.S. Pat. No. 5,013,532 hydrogen is used as a reducing agent and then the stream is humidified by a stream of water vapour to oxidize the metals and to recover the hydrogen.
There are also several systems for the recovery of Zn or Pb using a molten stage, including an electric arc furnace, an electrothermic smelting furnace and a slag fuming method. In U.S. Pat. No. 3,262,771, the recovery of Zn and Pb is carried out in a molten stage by using an electric arc furnace.
In U.S. Pat. No. 5,188,658, Zn and Pb are recovered from the molten stage in an electrothermic smelting furnace. The electrothermic smelting furnace described in U.S. Pat. No. 5,188,658 requires that the furnace be kept at high temperatures in order to maintain a volume ratio between CO2 and CO in the gas atmosphere in the smelting furnaces below 0.3.
In U.S. Pat. No. 3,017,261, Zn and Pb are recovered from the molten stage with a slag fuming method using a stationary furnace where metals are volatilized by melting the iron and steel dust completely and blowing air and reducing agent such as coal or coke into the molten iron. Because of higher temperature and operating conditions this processing offers great obstacles to the furnace operation.
There have also been attempts to render such industrial waste non-hazardous without recovering zinc or lead. In U.S. Pat. No. 4,840,671 electric arc furnace dust, steel dust from the production of certain specialty grades of steel, is rendered less hazardous by complexing the dust in a lime kiln dust, fly ash and hydrated lime mixture and then adding an aqueous solution containing ferrous hydroxide and calcium sulfate.